LibWeb: Parse the content property

For now, we only understand `none`, `normal`, `<image>` and `<string>`.
The various other functions and identifiers can be added later.

We can *almost* use a StyleValueList for this, except it's divided into
two parts - the content, and the optional "alt text". So, I've added a
new StyleValue for it.

Userland/Libraries/LibWeb/Forward.h

@@ -30,6 +30,7 @@ class BorderStyleValue;
 class BoxShadowStyleValue;
 class CalculatedStyleValue;
 class ColorStyleValue;
+class ContentStyleValue;
 class CSSImportRule;
 class CSSMediaRule;
 class CSSRule;